Please refer to FIG. 1, which is an embodiment of a flatbed-typed optical scanner typically seen in current market. Its main structure is that a document window glass 12 is arranged on the upper side surface of the outer casing 11 of an optical scanner 1 for supporting (placing) a manuscript to be scanned (not shown in the figure) and, bringing along an optical chassis 14 by a driving device 13 in the hollow outer casing 11 along a guiding rod 15 to proceed a linear motion, an image scanning job executed to the manuscript placed on the document window glass 12 is operated.
Please refer to FIG. 2, which is an A-A cross-sectional view for illustrating the optical chassis 14 of the optical scanner 1 according to the prior arts. The optical chassis 14 includes: a hollow outer casing 141, a light source 142 located at one appropriate position of the upper side surface of the outer casing 141, a light-guiding device comprised of plural reflection mirrors 143, a lens set 144, and a charge-coupled device 145 (CCD). The light, emitted from the light source 142, is incident onto the manuscript (not shown) placed on the document window glass 12, then the reflected light enters the casing 141 of the optical chassis 14 and is further reflected and direction-changed to increase its optical length to an appropriate length through the application of plural reflection mirrors 143 of the light-guiding device and, finally, the reflected light is focused by the lens set 144 and imaged on the charge-coupled device 145, by which the scanned image data is converted into electronic signal, wherein the total track (abbreviated as TT value) needed to focus the reflected light into a clear image is the summation of the lengths shown in FIG. 2: Y1+Y2+Y3+ . . . +Y5.
In the prior optical chassis 14 as shown in FIG. 1 and FIG. 2, since the lens set 144 is comprised of the elements such as convex lens etc. so, not only are the structure and the assembly of the lens set 144 all complicated and is the production cost higher, but also will the lens set 144 generate the effect of color-light separation to influence the scanning quality. Furthermore, since the reflection mirror 143 is comprised of a thin-plate-shaped glass, of which back is coated with silver, so the light will be reflected many times by plural glass mirrors during the reflection and the direction-changing procedure and, in such way, the effect of color-light separation will be strengthened. Besides, the reflection mirror 143 made of glass material is needed to be fixed at the predetermined position inside the casing 141 by additional spring piece 146, fixture mechanism, or other manners such as screw fixation so, not only are the assembly and the position more tedious, but also will the production cost be caused to increase substantially.